A Procedure to Estimate Absorption Rate of Sound Propagating Through High Temperature Gas

Abstract

A procedure to estimate the absorption rate of small disturbance acoustic waves propagating through high temperature gas due to relaxation is outlined. The procedure considers more than one relaxation mode, such as vibration modes and chemical reactions. It contains the known results for one relaxation mode as a special case. In the relaxation process, it is assumed that rotation and translation are in equilibrium and that vibration and chemistry may proceed at finite rates, while electronic excitation is assumed to be frozen in the equilibrium state. Some calculations are made as example cases with undisturbed base state in equilibrium for air, carbon dioxide and pure nitrogen in the temperature range from 1000K to 6000K. According to the results, carbon dioxide and air can have strong sound absorption due to vibrational relaxation at a frequency of around 1MHz and at a density ranging between 0.1kg/m^3 - 1.0kg/m^3, which are typical conditions of boundary layer transition experiments in the T5 hypervelocity shock tunnel. On the other hand, nitrogen does not have a strong absorption effect even at a very high temperature of 5000K.